Grommets made of synthetic resin such as shown in FIG. 9 have commonly been used when plate members are assembled or disassembled in a very limited space, such as in the case of fitting metal clips to a panel to secure a plate which is used as a foot rest provided beneath the door frame of an automobile.
The synthetic resin grommet comprises a female member A' which in turn comprises a cylindrical body portion 102 having at its upper portion a collar 101 that extends outwardly, and a male member B' which comprises a leg 103 to be inserted in the cylindrical body portion 102 of the female member A' and a head 104 to be fitted in said collar 101 of the female member A'. Plural slits 105, 105 are provided in the cylindrical body portion 102 extending from the tip end thereof toward the collar 101 to form plural elastic stopper means 102a. A catch 106 is provided on the inside of each elastic stopper means 102a at its lower end.
Dents 107 which receive the respective catches 106 of the stoppers 102a in the female member A' are provided on the leg 103 by segmenting the leg along its circumferential periphery. Each inclined surface between a dent 107 and the peripheral surface of the leg 103 in the direction of the head 104 is denoted as a slope 108. Horizontally stepped portions 109 are provided on the peripheral surface of the leg above the slopes 108, the stepped portions 109 being concentric with the leg.
Portions of the horizontal steps 109 are cut out in the longitudinal direction of the leg to form inclined surfaces 110.
As a result, when the male member B' is inserted in the female member A' which has been inserted in a fixing hole of a panel or the like, the elastic stopper means 102a of the female member A' are fitted first in the dents 107 of the male member B' and pressed outwardly by the slopes 108 of the male member B', while the catches 106 of the female member A' are held stopped at the horizontal steps 109. Thus, the panel is tightly held between the expanded elastic stoppers 102a and the collar 101 at the mouth of the fixing hole.
When a screw driver is inserted in the cross groove 111 of the male member B' to turn the male member B', projections 12 at the lower face of the head 104 of the male member B' move along the cam surfaces 113 provided in the collar 101 of the member A' and cause the male member B' to come off from the female member A'. By doing so, the intense pressing force of the catches 106 acting on the horizontal steps 109 is released while the catches 106 are guided toward the inclined surfaces 110.
By guiding the catches 106 from the horizontal steps 109 toward the inclined surfaces 110, the catches 106 snap fit into the dents 107 of the male member B' by their elastic resilience and return to the original position instead of being expanded outwardly, whereby the engagement of the grommet with the fixing hole of the panel is released.
Such prior art grommet is advantageous in that the grommet can easily and securely hold a panel at its fixing hole by inserting the female member A' and male member B' into said fixing hole, with the two members being coupled together, and by driving the male member B' further into the female member A'. By turning the male member B' pressed into the female member A', engagement between the grommet and the fixing hole of a panel is secured. On the other hand, because the horizontal steps 109 of the male member B' which receive the elastic stopper means 102a of the female member are concentric with the leg 103 and because the horizontal steps 109 are cut in the longitudinal direction of the leg to form inclined surfaces 110, it was somewhat difficult to hold the male member B' in firm engagement with the female member A' or to smoothly release their engagement.
In particular, because the horizontal steps 109 formed on the leg 103 of the male member B' are concentric with the leg 103, the pressing force generated between the catches 106 of the elastic stopper means 102a and the horizontal steps 109 would not change even when the male member B' is turned, having the male member B' turned with uniform force with respect to the female member A'.
This inconveniently causes spontaneous slackening of the engagement between the two members A' and B'. Moreover, as the cut surfaces 110 are inclined, the elastic stopper means 102a of the female member A' guided onto these surfaces 110 would not easily slide toward the dents 107; instead, they are caused to move by the elastic resilience of the stopper means 102a along the surfaces 110 while resisting against the latter.
Moreover, it is extremely difficult to accurately form the horizontal steps 109 which are concentric with the leg 103 and the inclined surfaces 110 which are obtained by cutting the steps 109 in the longitudinal direction of the leg, requiring highly skilled workers to form the mold for the male member B' and to remove the member from the mold.